1. Field of the Invention
The present invention relates to a lens system with a master lens system and a conversion lens system. More particularly, the present invention is directed to a lens system for varying a focal length of the lens system by inserting and removing a conversion lens system into and out of an optical path of the master lens system. The lens system in the present invention may be utilized as a photographing lens in a lens shutter camera and a video camera.
2. Description of the Background Art
Recently, there has been a strong demand for a popular camera designed as an integral unit of a camera body and a photographing lens system which can photograph at a wide-position and tele-position, etc., by varying a focal length of the photographing lens system.
In order to satisfy this demand, it has been proposed to design the photographing lens system for the popular camera as a zoom lens system capable of varying a focal length thereof or as a lens system capable of varying a focal length by inserting and removing a conversion lens into and out of an optical path of the photographing lens system.
As the lens system of the latter type, one example is disclosed in U.S. Pat. No. 4,708,442 which discloses a varifocal lens system. The varifocal lens system includes a master lens system, and an auxiliary lens group. The master lens system includes a front lens group having a positive refractive power and a rear lens group having a negative refractive power. The auxiliary lens group is arranged in such a manner that the auxiliary lens group can be freely inserted and removed between the front lens group and the rear lens group so as to vary a focal length of the overall varifocal lens system.
However, in the above-mentioned conventional varifocal lens system, a field angle of the master lens system is only 20 degrees. This deficiency can not minimize a bulk and size of the camera.
Further, when the auxiliary lens group is inserted between the front lens group and the rear lens group of the master lens system, the front lens group of the master lens system must be shifted to an image side.
Thus the varifocal lens system requires an undesirably complicated and large mechanism for varying a focal length.